Write all fields, terms and postings. This the "pull"
API, allowing you to iterate more than once over the
postings, somewhat analogous to using a DOM API to
traverse an XML tree.
You must compute index statistics,
including each Term's docFreq and totalTermFreq,
as well as the summary sumTotalTermFreq,
sumTotalDocFreq and docCount.
You must skip terms that have no docs and
fields that have no terms, even though the provided
Fields API will expose them; this typically
requires lazily writing the field or term until
you've actually seen the first term or
The provided Fields instance is limited: you
cannot call any methods that return
statistics/counts; you cannot pass a non-null
live docs when pulling docs/positions enums.